1. Field of the Invention
This invention relates to a method for recording/reproducing information utilizing near-field light and also to an information recording/reproducing apparatus that uses such a method.
2. Related Background Art
In recent years, information recording materials find applications at the very heart of the electronic industry that manufactures computers, computer-related apparatus, video discs, digital audio discs and other devices. In particular, various products using light for recording/reproducing information, such as magneto-optic discs, are enjoying an ever-increasing popularity.
Currently and in the foreseeable future, memory devices are required (1) to be able to store large volume of information with a high storage density, (2) to have a short response time for recording/reproducing information, (3) to be manufactured at low cost with a high productivity and (4) to consume power only at a reduced rate.
To improve the currently available optical-memory technologies and satisfy the above identified requirements, there has to be provided a technology that is adapted to down-sizing optical devices in the first place. Because of the fundamental property of diffraction on the part of light, optical devices show a dimensional lower limit that is defined by the wavelength of light to be used for the device. Additionally, any gauging system using optical devices cannot achieve a resolution exceeding the wavelength of light passing therethrough because it also utilizes the diffraction of light.
Under these circumstances, efforts have been paid recently to realize a gauging system and/or a fine machining system capable of dealing with dimensions as small as nanometers by utilizing fields (so-called evanescent fields) adapted to exploit electromagnetic interactions within a space dimensionally smaller than the wavelength of light passing therethrough. From the viewpoint of memory technologies, this technique is promising for high density recording/reproduction of information because it is adapted to store data within an area comparable to or smaller than the size of a molecule (or several nanometers) by utilizing and controlling the distribution of evanescent fields.
More specifically, scanning near-field optical microscopes (SNOMs) have been developed to observe the surface of an specimen by utilizing evanescent light seeping out from the surface of the specimen or the microaperture arranged at the tip of the minute probe of the microscope. Reports have been made, for example, on experiments of observing the emission of fluorescent light or changes in the optical transmissivity of a fluorescent material or a photochromic material in a minute local area comparable to a recording bit by utilizing such a microscope [E. Cohen et al., Phys. Rev. B25, 3828, (1982)].
FIGS. 1A and 1B of the accompanying drawings schematically illustrate the principle of a scanning near-field optical microscope. The mode of operation of the microscope as shown in FIG. 1A is referred to as C mode.
With a method of using a scanning near-field optical microscope, evanescent light (field) 73 seeping out to the surface of transparent substrate 71 when total reflection light is made to strike the specimen 72 on the substrate 71 from below is scattered by the specimen 72 to produce scattered light 74, which is measured by means of a probe 75 having a fine aperture at the front end thereof (transparent and coated with a metal thin film around the microaperture).
The mode of operation of the microscope in FIG. 1B is referred to I mode. In this mode of operation, scattered light 74 that is scattered by the specimen 72 when the specimen 72 is irradiated with evanescent light 73 seeping out from the probe 75 is detected by the same probe 75.
When an ordinary fluorescent light emitting pigment is used as the material of a recording medium, the recorded information can be lost as the pigment is discolored. When a photochromic material is used, there arises a problem of a low response speed and that of the lack of heat stability in isomerization.
There is a report of recording information on a Co/Pt recording medium [E. Betzig et al., Appl. Phys. Lett., 61, 142 (1992)]. However, the recording bit size of the recording medium of this report is as large as 100 nm, which is too large for high density recording.
The specimen or the recording medium has to be irradiated from below when the C mode detecting operation is used for reproducing information so that a transparent substrate has to be used to carry a specimen or a recording medium thereon.